In most of the aircraft made and in operation today, window frames made of aluminum are used, which comprise a part which is made by forging, truing and cupping. The window frame is organized into a total of three regions: an outer flange, an inner flange, and a vertical flange arranged perpendicular to and between these two flanges. The window frames are typically connected with two rows of rivets over the outer flange with the aircraft structure or with the exterior shell of the aircraft. A window element rests on the inner flange, which typically comprises two panes and a sealing arranged therebetween and which is fixed in its position via a retainer or downholder, which is connected with the window frame.
In addition to fixing the window element, such a window frame also has the function of absorbing the strain increase, which occurs on the edge of the comparably large cut-out for the window mounted in the load-transferring exterior shell. The outer flange of the window frame thereby serves, on the one hand, for reinforcement of this cut-out and on the other hand, via the outer flange, the frame and the exterior shell are connected to one another by means of rivets. Since the manufacture of the known aluminum window frame typically takes place by means of forging, it is not possible to achieve a cross-sectional distribution of the frame profile that is favorable for the rivet force distribution, since the slant of the flange may amount to a maximum of approximately two angular degrees, in order to enable a simple riveting.
The inner flange serves to receive the window element, whereby here a slanting of the mounting of the window is simplified. Simultaneously, the existing load from the interior pressure, which prevails in the passenger cabin, is transferred via this inner flange to the exterior shell of the aircraft.
The vertical flange serves usually exclusively as a reinforcement rib on the frame, in order to minimize the tension in the exterior shell with the least possible weight. On this vertical flange, also the eye bolts are attached, with which, typically, the downholder or retainer for the window elements are held in their position. At the same time, the vertical flange also forms the guide upon mounting of the window element.